In various display substrate glasses, particularly ones on whose surfaces a metal or oxide thin film is formed, the following characteristics have been conventionally required.
(1) Not substantially containing alkali metal ions; because in the case where an alkali metal oxide is contained, alkali metal ions diffuse in the thin film, resulting in deterioration of film characteristics.
(2) Having a high strain point so that deformation of a glass and shrinkage (thermal shrinkage) due to structure stabilization of the glass can be minimized when exposed to high temperature in a thin film formation step.
(3) Having sufficient chemical durability to various chemicals used in semiconductor formation; in particular, having durability to buffered hydrofluoric acid (BHF: mixed liquid of hydrofluoric acid and ammonium fluoride) for etching SiOx or SiNx, a chemical solution containing hydrochloric acid used for etching of ITO, various acids (nitric acid, sulfuric acid and the like) used for etching of an metal electrode, and an alkaline of a resist removing liquid.
(4) Having no defects (bubbles, striae, inclusions, pits, flaws, etc.) in the inside and on the surface.
In addition to the above requirements, the glass is in the following situations, in recent years.
(5) Reduction in weight of a display is required, and the glass itself is also required to be a glass having a small density.
(6) Reduction in weight of a display is required, and a decrease in thickness of the substrate glass is desired.
(7) In addition to conventional amorphous silicon (a-Si) type liquid crystal displays, polycrystal silicon (p-Si) type liquid crystal displays requiring a slightly high heat treatment temperature have come to be produced (a-Si: about 350° C.→p-Si: 350 to 550° C.).
(8) In order to improve productivity and increase thermal shock resistance by increasing the rate of rising and falling temperature in heat treatment for preparation of a liquid crystal display, the glass having a small average thermal expansion coefficient is required.
On the other hand, dry etching has prevailed, and requirement of BHF resistance has come to be weakened. As conventional glasses, many glasses containing B2O3 in an amount of from 6 to 10 mol % have been used in order to improve BHF resistance. However, B2O3 has a tendency to decrease the strain point. As examples of non-alkali glasses containing no or only small amount of B2O3, there are the following ones:
Patent Document 1 discloses a SiO2—Al2O3—SrO glass containing no B2O3. However, the temperature required for melting is high, which causes a difficulty in production.
Patent Document 2 discloses a SiO2—Al2O3—SrO crystallized glass containing no B2O3. However, the temperature required for melting is high, which causes a difficulty in production.
Patent Document 3 discloses a glass containing B2O3 in an amount of from 0 to 3% by weight. However, the strain point in Examples thereof is 690° C. or lower.
Patent Document 4 discloses a glass containing B2O3 in an amount of from 0 to 5 mol %. However, the average thermal expansion coefficient thereof at from 50 to 300° C. exceeds 50×10−7/° C.
Patent Document 5 discloses a glass containing B2O3 in an amount of from 0 to 5 mol %. However, the thermal expansion thereof is large, and the density thereof is also high.
In order to solve the problems in the glasses described in Patent Documents 1 to 5, a non-alkali grass described in Patent Document 6 is proposed. The non-alkali grass described in Patent Document 6 is considered to have a high strain point, to be able to be formed by a float process, and to be suitable for use in display substrates, photomask substrates and the like.
However, there is a solid phase crystallization method as a method for producing a high quality p-Si TFT. And in order to perform this method, it is required to further increase the strain point.
On the other hand, from a request in a glass production process, particularly melting and forming, it has been required to decrease viscous properties of the glass, particularly the temperature T4 at which glass viscosity reaches 104 dPa·s.